[From last April. Garmin GPS-USB-18 (no x).] I think I've figured out what's going on. This is reverse engineering (guessing) by looking at the output.
When the signal fades out, it remembers the most recent position and velocity. It does mark the following info packets as no-good. Yes, I could ignore the data, but I happened to plot it. It seems to update the position. Since this is my house, the velocity should be 0. Often it is. That's a boring case. It just keeps repeating the last position. But since the signal is fading out, the position is often quite a ways off. If the position on the previous slot was good, that turns into a large velocity. (When I started typing this in, it was off by more than 1 degree.) If the velocity has a N/S component and it doesn't recover the signal soon enough, it will eventually get to one of the poles. For me, that's happening several times a week. (I just noticed one that recovered at 88.464659.) It seems to be flying on a Mercator cylinder rather than a sphere. As you get closer to the poles, it still goes X meters per second E/W, but that gets translated into degrees which get smaller. If you plot latitude and longitude vs time, one is a straight line, the other turns into a curve. I've forgotten my spherical trig. What's the term for the change in direction as you fly a great circle route on a Mercator projection? This unit doesn't seem to be making that correction. It doesn't know how to fly over a pole. When it gets within a step of the pole, the N/S motion stops. It does know how to cross the international date line. I'm plotting the position every 64 seconds. As it gets near the pole, you get interesting aliasing effects as it wraps around the pole multiple times per step. -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
